The invention relates generally to X-ray optical elements and more particularly to multilayer thin film X-ray optical elements.
The demonstration of amplified spontaneous emission (ASE) at soft X-ray wavelengths has stimulated the need for normal incidence optics for soft X-rays. Amplification has been single pass amplified spontaneous emission and the amplifier, as well as its excitation and ionization, are produced by exploding a thin foil by interaction with a powerful optical laser. The exploding foil amplifier is coupled with various inversion schemes including neon-like and nickel-like collisional excitation as well as hydrogen-like three body recombination. In the case of Ne-like Se, the proper conditions for lasing are achieved by irradiating a 34 .mu.g/cm.sup.2 thick Se layer coated on one side of a 15 .mu.g/cm.sup.2 thickness plastic substrate with a pulse from the Nova laser operated at 0.53 microns, 500 ps FWHM pulselength, and 4.times.10.sup.13 W/cm.sup.2 intensity. The first high gain X-ray amplifier, at 20.6 and 20.9 nm, occurred in 1984. Since then using the neon-like schemes at least 15 laser transitions in Se, Y and Mo having wavelengths from 26.3 to 10.6 nm have been observed. Double pass amplification using a multilayer mirror operated at normal incidence has also been demonstrated. A multilayer X-ray mirror is placed at one end of a plasma X-ray amplifier to reinject one of the ASE amplifier's beams for further amplification.
From these results a natural direction for further X-ray laser advances is the development of an X-ray laser cavity. A cavity could provide significant enhancement of the X-ray laser emission and possible single transverse mode operation. Thus X-ray laser cavity components including normal incidence output couplers are required. An X-ray beam splitter which reflects and transmits some incident X-radiation is highly desirable; the reflected beam provides the necessary feedback into the laser cavity while the transmitted beam provides the output coupling. Thus the success of this next stage of X-ray laser research, the development of X-ray laser cavities, is highly dependent upon the availability of suitable cavity forming components including an output coupler such as an X-ray beam splitter. Unfortunately, although much research has been performed in the field of X-ray optics, an X-ray beamsplitter at soft X-ray and XUV wavelengths has not been heretofore available.
Multilayer coatings can be utilized as wavelength selective mirrors with reflectivities greater than 25%. As an example a multilayer mirror having about 20 layers of alternating Mo and Si with a layer periodicity of about 11 nm and mounted on a thick Si wafer can be used in the double pass cavity experiments. These X-ray mirrors are fabricated on a solid substrate material and produce X-ray reflection but no corresponding transmitted beam.
Conceptually these multilayer principles can be applied to X-ray beamsplitters as shown in "Current Developments in High Resolution X-ray Measurements", Attwood and Ceglio et al., Lawrence Livermore Laboratory UCRL-87540 (1982) and "Multilayer Structures for X-ray Laser Cavities", Ceglio et al., SPIE, Volume 563 Applications of Thin Film Multilayered Structures to Figured X-ray Optics (1985), pg 360. However, these conceptual beamsplitters were not successfully reduced to practice. Thus, a multilayer thin film X-ray beamsplitter has not been available.
U.S. Pat. No. 4,395,775 to Roberts shows a totally different type of beamsplitter design having a plurality of pores which transmit a portion of an incoming beam through the structure and a reflective surface surrounding the pores to reflect the remainder of the beam.
U.S. Pat. No. 4,317,043 to Rosenbluth shows an X-ray reflector having periodic monoatomic metal layers and hydrocarbon molecular layers. The X-ray reflector functions solely as a mirror and output coupling from the cavity is provided by a totally separate intracavity element such as a free standing foil which is placed at an angle to the cavity axis.
U.S. Pat. No. 3,991,309 to Hauer discloses a crystal which is stressed to enable and inhibit anomalous transmission therethrough.
Thus, although there is a need and interest in an X-ray beamsplitter, and multilayer theory could in principle be applied to design an X-ray beamsplitter, no one has successfully implemented a thin film multilayer X-ray beamsplitter. Such a beamsplitter could be used in a wide variety of applications, including interferometry and holography, as well as X-ray laser cavities.